1. Field of the Invention
The invention relates to a position-finding system for finding a position of a terminal, a server used in the position-finding system, a mobile terminal used in the position-finding system, a GPS receiver used in the position-finding system, a method of finding a position of a terminal in the position-finding system, and a program for causing a computer to carry out the method.
2. Description of the Related Art
In these years, a global positioning system (GPS) is mounted not only in devices used outdoors such as a car navigation system or a measurement unit, but also in a mobile terminal such as a mobile phone. With popularization of GPS, a device including GPS is required to have higher sensitivity and accuracy.
In a conventional method of finding a position of something through the use of GPS, a position was measured only with GPS. In contrast, there has been suggested a method of finding a position of something through the use of GPS, including the step of receiving preliminary or assistant data such as navigation message received from a satellite, through a network.
When a small terminal such as a mobile phone is used for finding a position thereof through the use of GPS, it is often impossible to capture a satellite, for instance, if the terminal is surrounded by buildings or the terminal is used indoors. Thus, it is requested that a position of a user of a terminal can be found, even if it is difficult for the terminal to capture a satellite.
In a method of finding a position of a terminal having GPS, four or more satellites are usually used. If a terminal can capture only three satellites, it would be necessary for the terminal to receive assist data comprised of data indicative of an altitude of the terminal, in which case, accuracy of the altitude data exerts serious influence on accuracy of a position found by the method.
A terminal having conventional GPS cannot receive altitude data. Hence, if the terminal can capture only three satellites, the terminal carries out two-dimensional position-finding on the assumption that an altitude of the terminal is equal to a predetermined altitude. Thus, if an actual altitude of the terminal is highly different from the predetermined altitude, a position of the terminal could be found with small accuracy, that is, the result of position-finding is accompanied with a significant error.
Recently, there have been suggested various methods of finding a position of a terminal at high speed by receiving assistant data from a server existing in a network.
For instance, Japanese Patent Application Publication No. 2002-351767 has suggested a method of displaying a contracted map in a display screen of a mobile terminal. The suggested method includes the steps of receiving positional data of the mobile terminal through the use of GPS, receiving a contracted map from a map server in accordance with the received positional data, and automatically selecting a scale of a map in accordance with accuracy of the positional data.
Japanese Patent Application Publication No. 4-370711 has suggested a method of finding an automobile through the use of GPS, including the steps of storing altitude data of nodes defining roads, into map data, receiving altitude data from the map data, corresponding to a position of an automobile detected by a navigation detector, if three-dimensional position-finding by GPS cannot be carried out, and calculating a current position of an automobile by carrying out two-dimensional position-finding through the use of the received altitude data.
Japanese Patent Application Publication No. 8-285932 has suggested a GPS receiver comprised of an antenna through which data signals are received from a GPS satellite, a demodulator for demodulating the received data signals, a digital map including altitude data, a reader for reading the altitude data out of the digital map, a calculator for calculating a position, a velocity vector and a current time of the GPS receiver, based on the demodulated data signals, the altitude data, and a difference between estimated time of the GPS receiver and time of the GPS satellite, a display for displaying the calculation results, and a unit for estimating time gap, based on data of past time gap. If data about time gap cannot be received from the GPS satellite, the unit compensates for the data received from the GPS satellite.
However, a fixed altitude is often used as altitude data in a base station or a cell, and accordingly, such fixed altitude is often quite different from an actual altitude in dependence on terrain.